In the conversion of light into electric current for the purpose of signal transmission, the requirements may vary and numerous realizations are possible. However, in almost every case, the conversion at a given signal bandwidth and intensity of the received light should be as noiseless as possible.
A component that is very frequently used for the conversion of light signals into electric signals is the photodiode. In many cases, it is superior to other converters. Since the electric signal delivered by a photodiode is normally very weak, it must be amplified by means of an amplifier in order to be utilized. At a given bandwidth and intensity of the received light, the photodiode and the amplifier largely determine the signal to noise ratio of the signal available at the output of the amplifier. Among the literature relating to the corresponding prior art, the book “Photodiode Amplifiers” by Jerald G. Graeme, McGraw-Hill, New York, 1996 may e.g. be cited.
A commonly used arrangement is shown in FIG. 1 and is discussed below.
U.S. Pat. No. 5,714,909 describes an amplifier with a feedback loop to provide a feedback current signal to cancel ambient noise in the current signal in the input node.
U.S. Pat. No. B1-6,329,881 describes a preamplifier with a feedback circuit including an average detecting circuit for detecting an average output voltage of the current feedback circuit and a current controlling circuit for distributing the input optical current in accordance with the detected average voltage.
U.S. Pat. No. B1-6,469,489 describes an electrical circuit with feedback means and a voltage-controlled current source.
U.S. Pat. No. 5,734,300 describes a control circuit for shunting DC and AC components of the signal from an electro-optical device away from a preamplifier input.
All the electrical circuits described in the US-patents mentioned above have the disadvantage that intense ambient light may cause a large shot noise resulting in a relatively poor signal to noise ratio.